This work was begun in October, 1959, as a population survey of rhesus monkeys in north central India. Regular census counts of rhesus in Aligarh District, U.P. are conducted three times a year (Feb.-Mar., June-July, Oct.-Nov.) in order to analyze the trends and dynamics of this population. Data are obtained on the numbers of groups in the study area, group sizes, sex and age ratios, birth rate, and mortality and loss rates. It is hoped that the project can be continued until 1979 to complete a 20 year record of this population.